1. Field of the Invention
The invention relates generally to the field of oil and gas well operations. More specifically, the present invention relates to a method and apparatus to facilitate severing sections of wellbore tubing.
2. Description of Related Art
Generally, a hydrocarbon producing wellbore consists of metal casing cemented into the surrounding formation and a production string that is inserted within the casing. The production string consists primarily of annular tubing and other completion members such as packers connected end to end to form a single annular member. The tubing, in combination with the packers, forms a conduit from the inside the wellbore through which oil and or gas is produced from the wellbore to the Earth's surface. Packers are employed inside of wellbores for various purposes, such as gravel packing, well control, well zone isolation, or to protect the casing.
Once placed inside of the wellbore, both the tubing and the packers are subject to mechanical failures. The tubing may begin leaking due to corrosion, material failure, or a faulty connection. One of the packers may develop a mechanical failure and no longer be able to provide its intended function. Depending on the severity of the failure, or the importance of the packer's function, it may be necessary to shut down the well and remove the damaged section of the production string for repair. Due to the structural configuration of the production string replacing and repairing the damaged section requires that the production string be severed below the damaged portion and that all of the production string above the damaged portion be removed from the wellbore.
The production string can be severed by use of a cutting device. Cutting devices can employ ballistics, torches, or extremely caustic chemical materials that radially emit a cutting medium and sever the production string at the desired location along its length. Because these incisions are often far below the Earth's surface, the ability to accurately position and stabilize these cutting devices from grade is very important. Accurate positioning of a cutting device is important to ensure that the proper section of the production string is removed from the wellbore. If the production string is severed well below the problem location, too much tubing or a functional packer may be inadvertently removed—which can be a loss of valuable equipment. Similarly, if the cut is errantly made above the problem location, an additional cut must be made to then remove the non-functioning portion of the production string to replace it with a properly functioning device. The added cutting step requires time and expense.
In some instances the thickness of the section to be severed is thicker than normal and more than one cutting sequence is required to completely sever the production string. Thus after the initial cut it would be required to raise the cutting tool from the wellbore, refurbish the cutting medium (i.e. chemical, pyrotechnics, fuel, etc.), and reinsert the cutting tool into the wellbore for additional cutting sequences until the production string is fully severed. Most cutting tools are suspended by wirelines, slicklines, or by some other line that is generally elastic and stretches during deployment. Because of the elastic lines combined with the weight of the cutting device, which can weigh in excess of 150 pounds, it can be difficult to position the cutting device to the exact depth of a previous cut.
Magnetic positioning devices that monitor thickness variations of the tubing string, such as collar locators, can be deployed along with the cutting tool to aid in positioning the cutting tool to the proper depth for cutting. However, the production string often comprises a multitude of packers whose thickness can vary along their lengths or vertical axis. This can produce confusing thickness measurements, which in turn makes positioning of the cutting device difficult and uncertain.
During the cutting process large forces are produced by the cutting medium (chemical, pyrotechnic, or flame) that are exerted onto the cutting device. The forces are so severe that they can cause the cutting device to move during the cutting process. This is highly undesired since movement of the cutting device during the cutting sequence will result in uneven jagged cuts, or in some cases may not result in a complete cut. To ensure that the cutting device is secured within the casing during the cutting process, numerous brakes or braces have been developed. One of the braking or bracing means developed involves placing an anchor at some predetermined point within the casing upon which the cutting tool or perforating device will rest. Another embodiment involves a series of fins that expand outward from the cutting device and engage the casing prior to initiating the cutting procedure to secure the tool to the casing during the cutting procedure. One device involves a pyrotechnic cutting tool that equalizes the gases exiting from the cutting device above and below the cutting flame exit point such that the pressure of the exiting gas will stabilize the tool within the casing. The equal pressures purports to preclude upward or downward movement of the cutting device during the cutting process. Yet another embodiment involves plugs having serrated surfaces that also extend outward from the cutting device to prevent axial movement during the cutting procedure. Examples of these devices can be found in U.S. Pat. No. 6,223,818 issued to Hrupp; U.S. Pat. No. 6,186,226, issued to Robertson; U.S. Pat. No. 5,435,394 issued to Robertson; and U.S. Pat. No. 4,598,769 issued to Robertson. However, each of these devices suffers from one or more of the following disadvantages.
One of the problems associated with the device disclosed in the above noted patents is it is difficult or impossible to position the cutting device at the precise depth within the wellbore to sever the production string at the desired location. These tools rely on some type of line to be deployed into a wellbore. As noted above, when the section of production string to be cut is far below the earth's surface, utilization of an elastic line introduces a potential for vast measurement error when positioning the cutting device prior to its cut. Further, although the above noted devices took advantage of frictional forces existing between the slips or serrated edges contacting the casing, these devices can still become dislodged during the cutting process. Therefore, there exists a need for a device useful for severing pipe within a wellbore, where the device can be precisely located within the wellbore for at least one cutting sequence; and during cutting the device remains secure within the casing.